This invention relates to an improved electrochromic medium capable of producing a pre-selected color and, more particularly, an improved electrochromic device having an electrochromic medium comprising at least three electroactive materials whose concentrations may be chosen to produce a pre-selected perceived color, where the electrochromic medium generally maintains the pre-selected perceived color throughout its normal range of voltages when used in an electrochromic device.
Electrochromic devices have been proposed for commercial applications for nearly seventy years (British Patent Specification No. 328,017 (1929) to F. H. Smith). However, the first commercially successful electrochromic device, a dimmable rearview mirror for motor vehicles, was not introduced until 1987. Various automatic rearview mirrors for motor vehicles have been devised which automatically change from the full reflectance mode (day) to the partial reflectance mode(s) (night) for glare protection purposes from light emanating from the headlights of vehicles approaching from the rear. The electrochromic mirrors disclosed in U.S. Pat. No. 4,902,108, entitled "Single-Compartment, Self-Erasing, Solution-Phase Electrochromic Devices, Solutions for Use Therein, and Uses Thereof", issued Feb. 20, 1990 to H. J. Byker; Canadian Patent No. 1,300,945, entitled "Automatic Rearview Mirror System for Automotive Vehicles", issued May 19, 1992 to J. H. Bechtel et al.; U.S. Pat. No. 5,128,799, entitled "Variable Reflectance Motor Vehicle Mirror", issued Jul. 7, 1992 to H. J. Byker; U.S. Pat. No. 5,202,787, entitled "Electro-Optic Device", issued Apr. 13, 1993 to H. J. Byker et al.; U.S. Pat. No. 5,204,778, entitled "Control System For Automatic Rearview Mirrors", issued Apr. 20, 1993 to J. H. Bechtel; U.S. Pat. No. 5,278,693, entitled "Tinted Solution-Phase Electrochromic Mirrors", issued Jan. 11, 1994 to D. A. Theiste et al.; U.S. Pat. No. 5,280,380, entitled "UV-Stabilized Compositions and Methods", issued Jan. 18, 1994 to H. J. Byker; U.S. Pat. No. 5,282,077, entitled "Variable Reflectance Mirror", issued Jan. 25, 1994 to H. J. Byker; U.S. Pat. No. 5,294,376, entitled "Bipyridinium Salt Solutions", issued Mar. 15, 1994 to H. J. Byker; U.S. Pat. No. 5,336,448, entitled "Electrochromic Devices with Bipyridinium Salt Solutions", issued Aug. 9, 1994 to H. J. Byker; U.S. Pat. No. 5,434,407, entitled "Automatic Rearview Mirror Incorporating Light Pipe", issued Jan. 18, 1995 to F. T. Bauer et al.; U.S. Patent No. 5,448,397, entitled "Outside Automatic Rearview Mirror for Automotive Vehicles", issued Sep. 5, 1995 to W. L. Tonar; and U.S. Pat. No. 5,451,822, entitled "Electronic Control System", issued Sep. 19, 1995 to J. H. Bechtel et al., each of which patents is assigned to the assignee of the present invention and the disclosures of each of which are hereby incorporated herein by reference, are typical of modem day automatic rearview mirrors for motor vehicles. Such electrochromic mirrors may be utilized in a fully integrated inside/outside rearview mirror system or as an inside or an outside rearview mirror system. In general, in automatic rearview mirrors of the types disclosed in the above referenced U.S. Patents, both the inside and the outside rearview mirrors are comprised of a relatively thin electrochromic medium sandwiched and sealed between two glass elements.
In most electrochromic mirrors, when the electrochromic medium which functions as the media of variable transmittance is electrically energized, it darkens and begins to absorb light, and the more light the electrochromic medium absorbs the darker the mirror becomes. When the electrical voltage is decreased to zero, the mirror returns to its clear state. The electrochromic medium is contained in a sealed chamber defined by a transparent front glass element coated with a transparent conductor, a peripheral edge seal, and a rear mirror element having either a reflective layer or a transparent conductive layer in contact with the electrochromic medium depending on whether the mirror has a third or fourth surface reflector. The conductive layers on both the front glass element and the rear glass element are connected to electronic circuitry which is effective to electrically energize the electrochromic medium to switch the mirror to nighttime, decreased reflectance modes when glare is detected and thereafter allow the mirror to return to the daytime, high reflectance mode when the glare subsides, as described in detail in the aforementioned U.S. Patents. For clarity of description of such a structure, the front surface of the front glass element is referred to as the first surface, and the inside surface of the front glass element is referred to as the second surface. The inside surface of the rear glass element is referred to as the third surface, and the back surface of the rear glass element is referred to as the fourth surface.
The electrochromic medium is typically comprised of solution-phase electrochromic materials, electrodeposition type electrochromic materials, surface confined electrochromic materials or combinations thereof. The electrochromic medium changes from a clear or high visible light transmission level, to a lightly colored state, to a moderately colored state, and to a dark or low visible light transmission colored state when various voltages are applied and electrochemical oxidation and reduction take place. An important factor in determining the desirability of an electrochromic device is its perceived color when in its clear state and dark state and any state therebetween. The perceived color of an electrochromic mirror includes the influences from the front glass element, the two transparent conductive coatings, the reflector and, most importantly, the electrochromic medium.
Generally speaking, there is a desire for a gray colored electrochromic medium in interior mirrors and most exterior mirrors for motor vehicles because the perceived color of the reflected image will closely resemble the color of the object before being reflected. In addition, it is desirable that the electrochromic device maintain this gray color during its darkening and clearing transitions so that the perceived colors of a reflected image do not change during these transitions. However, arguments have been made for tinted or colored mirrors. For example, commonly assigned U.S. Pat. No. 5,278,693 to D. A. Theiste et al., discloses adding an electrochemically inactive and stable compound to a solution-phase electrochromic device to provide a blue tint. This electrochemically inactive compound is essentially a dye normally present at low levels, and will provide a perceived tint to the device only in the highest reflectance or transmittance states when little or no voltage is applied.
In other applications such as architectural windows, sun roofs, displays and specialty windows, various colors (e.g., blue, greens, purples, yellows) in addition to gray may be desirable for a number of reasons. For instance, it may be desired that certain electrochromic windows be tinted to match the decor of the room, provide contrast enhancement or gray scale dimming filters for displays emitting particular colors of light, or to give a building a particular color or appearance.
A problem in the art, then has been the inability to pre-select a color of an electrochromic device while simultaneously ensuring that the device generally maintains the desired color when in its clear state and dark state and any state therebetween. With such devices used as electrochromic rearview mirrors for motor vehicles and many window applications, a desired color is one that is perceived as gray. For other applications, colors that are perceived other than gray, (e.g., red, yellow, green, blue, purple) may be desirable.
Consequently, it is desirable to provide an improved electrochromic medium having at least three electroactive materials whose concentrations may be chosen to produce a pre-selected perceived color, where the electrochromic medium generally maintains the pre-selected perceived color throughout its normal range of voltages when used in an electrochromic device.